Various assemblies have been devised to run an electrical motor for a predetermined period of time, to move a device from a first position to a second position. These assemblies are also devised to allow the motor to run in the opposite direction, to move the device from the second position back to the first position.
Assemblies of this type are used to control many different devices that run on a DC motor and require bi-directional movement. These assemblies can be used to control the initial and final positions of various electric devices, such as those used in medical equipment, automation production and motor vehicles. Examples of uses of these assemblies in automobiles include, for example, control circuitry for side-view mirrors, convertible tops and retractable head lights.
Movement of the devices between the initial and final positions is usually effected by a small electric motor that drives a gear mechanism. When used in automobiles, these motors are usually sub-fractional horsepower DC motors that operate from the 12-volt vehicle power system. Applying the voltage in one polarity causes the motor mechanism to operate in one mode of the initial/final position and reversing the polarity effects the alternate mode.
For many devices in the automobile industry, designers prefer to provide a single snap-action two-state switch for the vehicle operator to select the device deployment mode. The switch is a snap-action switch that retains the selected setting after the operator removes his or her finger. The switches are usually a dual-pole, double throw (DPDT) type that are cross wired to provide the 12-volts in either polarity orientation to the DC motor, and thus alternate the operating direction of the motors. Other switches that provide alternating polarity to the circuit could also be used, including mechanical switches of the sliding or push button variety, appropriately configured contacts of electric relays, and rotary switches with appropriate contact geometry.
What is required for these powering topologies is an intervening circuit between the switch and the DC motor to remove power to the motor after a sufficient time has passed to allow the initial/final position function to be completed. Without this intervening circuit, voltage would be applied to the DC motor continuously, even after the mechanism had completed implementing the selected deployment mode. The continual application of voltage to the stalled motor would be unacceptable, due to resulting damage to the motor and depletion of battery energy. The continuous application of voltage to a stalled motor significantly increases the current drawn by the motor, causing the motor to become overheated and possibly damaged. Moreover, when used in an automobile, application of voltage to a motor at all times, even when the automobile is parked, will result in draining the battery.
Various assemblies have been devised to cut off electrical power to a motor after it has driven a device for a predetermined period. U.S. Pat. No. 5,703,732 to Boddy et al. refers to an electrical control system for a mirror unit that uses an RC timer circuit. The timer is comprised of two resistors, two diodes and a single capacitor. The capacitor and one of the resistors form an RC circuit causing a gradual rise in voltage. This rising voltage eventually turns off a transistor, which turns off a relay, which turns off the motor. U.S. Pat. No. 4,524,312 to Matsumoto refers to a control system that uses a single capacitor, a driver transistor and a power transistor. Power is supplied to the DC motor when the power transistor becomes conductive. Power is shut off after the capacitor is gradually charged up so as to render the driver transistor conductive. In response to the conduction of the driver transistor, the power transistor becomes nonconductive, to shut off power to the motor.
Other known control circuits require mechanical movement for controlling the motor. U.S. Pat. No. 5,315,442 to Sato et al. refers to a control system for an electrically powered foldable mirror that includes a mechanical stop device and a sensor for detecting a halt to motor rotation. A control circuit means interrupts the motor drive circuit when the sensor detects the halt of the motor rotation. U.S. Pat. No. 4,657,362 to Suzuki also refers to a control system for an electrically foldable mirror. When the mirror visor reaches the closed position, a movable contact member comes into contact with a terminal causing a capacitor to discharge and turning off transistors. Thus, the motor is de-energized, causing the motor to stop.
U.S. Pat. No. 4,403,178 to Kaminski refers to two different circuit strategies for the bi-directional control of a DC motor for gear shifting on a truck. The circuit disclosed in FIG. 2 uses a complicated user-operated mechanical switch to connect and disconnect one capacitor at a time to activate one of two sides of an H-bridge transistor power switch. While one capacitor is connected via the switch to turn on its side of the H-bridge, the other capacitor is removed from its side of the H-bridge and allowed to recharge. The circuit disclosed in FIG. 4 uses two timing capacitors and resistors for controlling the length of time that the motor is activated in either direction. The mechanical switch is used to alternately apply the same polarity of voltage to one capacitor-resistor combination or the other. Each of the United States patents cited hereinabove is hereby incorporated by reference into the present specification.
Disadvantages of known control circuits include the use of expensive and bulky solid state electronic elements.
Another disadvantage of known control circuits is their reliance on moving mechanical parts. As often occurs in devices with moving parts, these circuits are susceptible to malfunctioning.
Another disadvantage of known control circuits includes the drawing of significant current when the control circuit is not powering the motor. Control circuits that require continuously forward biasing a driver transistor will draw significant current which can drain the battery.
Another disadvantage of known control circuits includes the inability to easily provide alternate activation times for the motor for each operating direction of the motor mechanism.